1. Field of the Invention
The present invention relates to a semiconductor device having an isolation layer and a manufacturing method thereof, and more particularly, to a semiconductor device having a shallow trench isolation structure and a manufacturing method thereof.
2. Description of the Related Art
With the advancement of semiconductor device manufacturing techniques, the speed and integration of semiconductor devices has improved. In addition, small, high-density patterns have been increasingly required. Wide isolation regions in semiconductor devices also require small high density patterns.
Local oxidation of silicon (LOCOS) oxide layers have been mainly used as conventional isolation layers of semiconductor devices. However, bird""s beak configurations are created at the edges of the isolation layers by the LOCOS method and thus the area of active regions is reduced, and current leakage occurs.
Presently, shallow trench isolation (STI) layers having narrow widths and excellent isolation characteristics are widely used.
Referring to FIG. 1, a blocking pattern (not shown) is formed on a semiconductor substrate 10 to expose an isolation region. The semiconductor substrate 10 is defined as a cell area, a core area and a peripheral area. In addition, the blocking pattern may be a stack layer comprising an oxide layer and a silicon nitride layer. The exposed semiconductor substrate 10 is etched to a predetermined depth using the blocking pattern as a mask, thereby forming trenches t1 and t2 therein. Herein, the trench t1 may be formed in the cell area and the trench t2 may be formed in the core and peripheral areas. The etching process for forming the trenches t1 and t2 is performed by a dry etching method using a plasma.
The dry etching process for forming the trenches t1 and t2 may cause silicon lattice defects and damage to the inner surfaces of the trenches t1 and t2.
Conventionally, to reduce silicon lattice defects and damage, a sidewall oxide layer 12 is formed by thermally oxidizing the inner surfaces of the trenches t1 and t2. At this time, the sidewall oxide layer 12 is formed to a thickness of only 50 to 100 xc3x85. Also, the formation of the sidewall oxide layer 12 helps the removal of sharp upper and lower corners of the trenches t1 and t2.
Afterwards, a silicon nitride liner 14 is formed on the surface of the sidewall oxide layer 12. The silicon nitride liner 14, as is well known, prevents the generation of stress due to a difference in thermal expansive coefficients of the semiconductor substrate 10 made of silicon and a silicon oxide layer that will be filled into the trenches t1 and t2.
A dielectric material, for example, a high density plasma (hereinafter, referred to as xe2x80x9cHDPxe2x80x9d) oxide layer is deposited over the resultant semiconductor substrate 10 to fully fill the trenches t1 and t2. Next, chemical mechanical polishing (hereinafter, referred to as xe2x80x9cCMPxe2x80x9d) is performed on the HDP oxide layer and the blocking pattern to expose the surface of the semiconductor substrate 10, thereby filling the trenches t1 and t2 with the HDP oxide layers. Consequently, a shallow trench isolation (STI) layer 16 is completed.
However, forming the thin and uniform sidewall oxide layer 12 causes the following problems. With reference to FIGS. 2A and 2B, since hot carriers of a highly integrated semiconductor MOS transistor generally have high energy, they bounce to a thin gate oxide layer 22 or easily penetrate through the sidewall oxide layer 12 into the STI layer 16. Herein, the hot carriers penetrating into the STI layer 16 are mainly negative electric charges, namely, electrons 100, which are easily trapped in the silicon nitride liner 14 and on the interface between the silicon nitride liner 14 and the sidewall oxide layer 12. The electrons 100 are densely trapped since the sidewall oxide layer 12 is remarkably thin as mentioned above. If the electrons 100 are densely concentrated around the edge of the STI layer 16, positive electric charges in semiconductor substrate 10 on which MOS transistors are formed, namely, holes 12 are gathered in the periphery of the STI layer 16. At this time, since the electrons 100 are densely trapped in the silicon nitride liner 14 and on the interface between the silicon nitride liner 14 and the sidewall oxide layer 12, the holes 12 in the semiconductor substrate 10 are densely gathered together.
Herein, as shown in FIG. 2A, since in an N-channel field effect transistor (N-FET) the major carriers are the electrons 100, a path is not formed between n-type junction areas 26a and 26b in which the electrons 100 function as major carriers, even though the holes 102 are dense in the periphery of the STI layer 16.
Because in a P-channel field effect transistor (P-FET) the major carriers are the holes 102, as shown in FIG. 2B, the holes 102 densely arranged at the periphery of the STI layer 16 function as a current path I connecting p-type junction areas 28a and 28b isolated by the STI layer 16. Consequently, due to the current path I, although p-type junction areas 28a and 28b are isolated by the STI layer 16, leakage current, such as abnormally increased standby current after burn-in, is generated between adjacent P-FETs, thereby deteriorating the device characteristics of the P-FETs. Herein, a reference numeral 24 denotes a gate electrode of a MOSFET.
Furthermore, in a case where a P-FET is on the interface between the STI layer 16 and an active region (hereinafter, referred to as xe2x80x9cinterfacexe2x80x9d), a channel area of the P-FET (not shown) is opposite to the silicon nitride liner 14 where the electrons are trapped. Here, the thin sidewall oxide layer 12 is interposed between the channel area of the P-FET and the silicon nitride liner 14. Consequently, the electrons trapped in the silicon nitride liner 14 easily induce holes in the channel area of the P-FET on the interface. And, the holes induced in turning on the P-FET are not easily removed and remain after turning off the P-FET. Due to this, the length of the channel of the P-FET on the interface is gradually reduced, thereby changing the threshold voltage. Consequently, the characteristics of the P-FET are changed.
To solve the above problems of the P-FET, techniques for increasing the entire thickness of the sidewall oxide layer 12 have been proposed. However, if the entire thickness of the sidewall oxide layer 12 is increased, oxidants easily penetrate into the sidewall oxide layer 12. Due to the penetration of such oxidants, stress in the N-FET in the cell area connected to a storage capacitor is increased thereby sharply reducing data retention time of the storage capacitor, namely, refresh time. Consequently, the characteristics of a DRAM device are deteriorated.
In summary, if the sidewall oxide layer 12 of the STI is formed to a uniform thickness throughout the entire area, which does not generate abnormally increased standby current after burn-in in the P-FET, then the standby current after burn-in of the P-FET as well as the data retention time of the storage capacitor in the cell area is reduced. If the sidewall oxide layer of the STI is formed to a uniform thickness throughout the entire area, which maintains moderate data retention time of a DRAM device, then the data retention time of the DRAM is maintained while serious abnormally increased standby current after burn-in is generated in the P-FET. Consequently, it is difficult to maintain the characteristics of the P-FET.
Consequently, if sidewall oxide layers in their respective areas are formed to a uniform thickness, it is difficult to simultaneously maintain the excellent device characteristics of the N-FET in the cell area and of the P-FET in the core and periphery areas.
To solve the above problems, it is an object of the present invention to provide a semiconductor device having a shallow trench isolation (STI) structure, which is capable of reducing abnormally increased standby current after burn-in in a P-FET, maintaining the device characteristics of the P-FET and improving the characteristics of a memory device such as a DRAM device.
It is another object of the present invention to provide a method of manufacturing the semiconductor device having the STI structure.
Accordingly, to achieve the first object, there is provided a semiconductor device having a shallow trench isolation (STI) structure, comprising a semiconductor substrate having a first area with a first trench formed therein and a second area with a second trench formed therein, a first sidewall oxide layer formed on the inner surface of the first trench, a second sidewall oxide layer, which is thinner than the first sidewall oxide layer, formed on the inner surface of the second trench, a liner formed on the surfaces of the first and second sidewall oxide layers, and a dielectric material with which the first and second trenches are filled.
To achieve the second object, there is provided a semiconductor device having an STI structure, comprising a semiconductor substrate having core and periphery areas in which a P-FET and other circuit devices are formed, a cell area in which memory devices are formed, and first and second trenches for isolating devices formed in the cell area and core and periphery areas, a first sidewall oxide layer formed on the inner surface of the first trench, a second sidewall oxide layer, which is thinner than the first sidewall oxide layer, formed on the inner surface of the second trench, a liner formed on the surfaces of the first and second sidewall oxide layers, and a dielectric material with which the first and second trenches are filled, wherein the first trench is formed in the core and periphery areas and the second trench is formed in the cell area. Also, the first trench may be formed in an area for dividing P-FETs in the core and periphery areas and the second trench may be formed in the cell area and in an area for dividing N-FETs, an N-FET and a P-FET, an N-FET and other circuit devices, a P-FET and other circuit devices, and other circuit devices in the core and periphery areas. The first sidewall oxide layer has a thickness capable of preventing a significant increase in standby current after burn-in in the P-FET. The second sidewall oxide layer has a thickness that does not appreciably reduce a predetermined data retention time of a memory device.
According to the second preferred embodiment of the present invention, there is provided a method of manufacturing a semiconductor device having an STI structure. In the method, a first trench and a second trench are formed in selective areas of a semiconductor substrate. A first sidewall oxide layer is formed on the inner surface of the first trench and a second sidewall oxide layer is formed on the inner surface of the second trench. The first and second trenches are filled with a dielectric material. It is preferable that the second sidewall oxide layer be thinner than the first sidewall oxide layer.
According to the third preferred embodiment of the present invention, there is provided a method of manufacturing a semiconductor device having an STI structure. A semiconductor substrate having core and periphery areas in which a P-FET and other circuit devices are formed and a cell area in which a memory device is formed is provided. A first trench and a second trench are formed in the core area, the periphery area, and an area for device isolation in the cell area of the semiconductor substrate. An initial oxide layer is formed on the inner surfaces of the first and second trenches. The initial oxide layer in the second trench is removed. First and second sidewall oxide layers are formed on the inner surfaces of the first and second trenches by oxidizing the initial oxide layer in the first trench and the inner surface of the second trench. The first and second trenches are filled with a dielectric material. The first sidewall oxide layer is thicker than the second sidewall oxide layer.
According to the fourth preferred embodiment of the present invention, there is provided a method of manufacturing a semiconductor device having an STI structure. In the method, a semiconductor substrate having core and periphery areas in which a P-FET and other circuit devices are formed and a cell area in which a memory device is formed is provided. A first trench and a second trench are formed in the core area, the periphery area and a pre-isolation area in the cell area of the semiconductor substrate. A first sidewall oxide layer is formed to a predetermined thickness on the inner surfaces of the first and second trenches. A second sidewall oxide layer is formed by etching the first sidewall oxide layer in the second trench to a predetermined thickness. The first and second trenches are filled with a dielectric material. The first trench is formed in the core and periphery areas and the second trench is formed in the cell area. The first trench may be formed in an area for diving P-FETs in the core and periphery areas and the second trench may be formed in the cell area and in an area for diving N-FETs, an N-FET and a P-FET, an N-FET and other circuit devices, and a P-FET and other circuit devices in the core and periphery areas. Preferably, the first sidewall oxide layer in the first trench is formed to a thickness capable of preventing a significant increase in standby current after burn-in in the P-FET, and the second sidewall oxide layer is formed to a thickness that does not appreciably reduce a predetermined data retention time of a memory device.